Refactor async tests

[fealebenpae]

This is a (noisy) refactor of our unit tests that extracts the logic behind TestHelpers.RunAsyncTest and SyncTestHelpers.RunBaasTestAsync into two custom NUnit attributes - [RequiresSynchronizationContext] and [RequiresBaas]. This allows us to switch to using awaitable test methods instead of wrapping async code in TestHelpers.RunAsyncTest() calls.

I originally intended to just replace SyncTestHelpers.RunBaasTestAsync calls with the [RequiresBaas] attribute and remove TestHelpers.RunAsyncTest in favor of making the test methods async themselves, but it turned out that NUnit doesn't automatically install a SynchronizationContext and the only way to tell it to do so is to mark your tests as needing a single-threaded apartment, which also causes NUnit to try to run tests inside a STA thread which .NET only supports on Windows. As a workaround I ended up creating [RequiresSynchronizationContext], made it inheritable, and slapped it on our base fixture class RealmTest, so while it exists, there shouldn't be a need to use it in test code if the test class inherits from RealmTest.

For all intents and purposes now it's enough for a test method to return Task (or another awaitable type) to be considered async, and to decorate any baas integration tests with [RequiresBaas].

[nirinchev]

Good stuff - I like this a lot better than the current approach.

[papafe]

Looks quite cool!

[papafe]

Why don't use the synchronization context that's already there if it exists?

[DominicFrei]

Nice one. Big fan of simplifications! 👍

[LaPeste]

As we discussed already, for this and all other usage we'll need to re-adjust this once the client reset API PR gets merged as the Session.Error is deprecated from there on

[LaPeste]

Really good work. It's gonna make our tests so much prettier in style and faster to read and write.

[coveralls]

Pull Request Test Coverage Report for Build 2016072633

• 8 of 9 (88.89%) changed or added relevant lines in 1 file are covered.
• 6 unchanged lines in 3 files lost coverage.
• Overall coverage increased (+0.2%) to 83.201%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
Realm/Realm/Native/SynchronizationContextScheduler.cs	8	9	88.89%

Files with Coverage Reduction	New Missed Lines	%
Realm/Realm/Extensions/TaskExtensions.cs	1	81.25%
Realm/Realm/Exceptions/RealmException.cs	2	39.45%
Realm/Realm/Handles/SessionHandle.cs	3	85.06%

Totals
Change from base Build 2001531696:	0.2%
Covered Lines:	5564
Relevant Lines:	6572

---

💛 - Coveralls